This invention relates to liquid crystal display devices (LCDs) which are compatible with night vision goggles (NVGs). More particularly, this invention relates to backlit, liquid crystal display devices which filter out infrared light, thereby achieving compatibility with infrared-sensitive night vision goggles used in avionics and for military purposes.
The use of night vision goggles on flight decks and for many other military and avionics applications is rather widespread and increasing. Such goggles operate successfully because they are highly sensitive to very low levels of light, mainly in the near infrared (IR) region of the spectrum (i.e. about 630-1100 nm). The use of NVGs is not without its problems, however. For example, a major problem occurs in airplane cockpits when stray light from panel equipment and displays (e.g. an LCD) is reflected into the NVG, saturating it. Despite a significant effort undertaken in the prior art to block the IR region of the displays and panel equipment from emitting any IR light, particularly in the near IR region, this effort has not been rewarded with true success. This is because to be truly successful IR blockage must be accompanied by maintenance of color integrity (particularly of the color red) and the ability to view the LCD at reasonably wide angles from normal (e.g. up to about 60.degree.). This is particularly true when it comes to achieving these results in the highly advantageous display devices known as backlit, liquid crystal displays (e.g. of the active matrix type, "AMLCD").
The problems associated with achieving acceptable IR blockage, while maintaining color integrity and wide viewing angles, are reported and demonstrated in Abileah et al., "A Full Color AML With NVG Class B Compatibility" IEEE, AES Magazine (March, 1992) pp. 1237 thru 1241. The entire disclosure of this report is incorporated herein by reference. It discloses not only some of the background from which this invention arose, but a preferred mode of this invention which solved the problem in the prior art as described above.
Generally speaking, the problem of achieving acceptable blockage of IR light, while maintaining color integrity and a reasonably wide viewing angle, is not simply achieved by using an IR-absorbing filter The major problem with the use of such a filter is that most known filters cut off some of the visible red light region as well (e.g. FIG. 2, Abileah et al., supra). This lack of a sharp cutoff between near IR and visible red results in an unbalanced white color, shifting the red color towards the orange. The resulting display thus can not pass the NVIS-B criteria of the MIL-L-857624 Standard.
The obvious solution, of course, would be to simply choose a filter with a sharp, near IR/red cutoff point. The problem is that known filters with such a sharp cutoff point only manifest this behavior (or characteristic) at an angle substantially normal to or at some preselected angle to the filter. As one views the LCD from angles more inclined from normal, the transmission curve once again exhibits the aforesaid problem of losing a sharp outoff between near IR and visible red A tri-peak backlight, for example, will shift into the cyan region because the red transmission is cut from the white balance. True success, therefore, is not achieved by taking the obvious step of merely using a sharp cutoff filter.
In co-pending application Ser. No. 514,737, having an overlapping inventorship herewith, now U.S. Pat. No. 5,161,041, there is disclosed a unique LCD and backlighting system therefore The entire disclosure of this application/patent is incorporated herein by reference. Generally speaking, this unique LCD improved upon prior art backlit LCDs by including in such a system an integral collimating and image splitting means for collimating light from the light source in the LCD and for refracting light rays emanating from this light source to provide two, similar images thereof. By locating the split images contiguous, one to each other, the area of illumination was not only effectively enlarged, but a bright, uniform light distribution across a low profile LCD also resulted. This also resulted in wide angle viewability as well, by providing a light diffuser between the integral collimator/image splitter and the matrix array. The precise diffuser chosen depends on the application of the LCD.
In preferred forms of that invention the integral collimating and image splitting means included a thin film having light-refracting, faceted prisms formed on one of its faces. An example of such a film is 3M SCOTCH.TM. Optical Lighting Film. In preferred forms, this thin 3M SCOTCH. film is used by laminating it to a clear transparent sheet of glass, ceramic or plastic, and thereafter used as a layer in a low profile LCD stack. As will be described more fully below, by employing the basic concepts of this aforesaid co-pending invention and coupling it with a sharp cutoff IR filter of the requisite type, the instant invention unexpectedly is able to provide a true solution to the problem of NVG compatibility.